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REGULATORY MATTERS - Narrative (Details) - USD ($) | Mar. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
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Common equity tier 1 capital to risk-weighted assets: |  |  |  |  |
Capital conservation buffer percentage, increase |  | 0.625% | 0.625% | 0.625% |
Subordinated debentures | Subordinated Debentures II, Subordinated Debentures III, and DCB Debentures I |  |  |  |  |
Common equity tier 1 capital to risk-weighted assets: |  |  |  |  |
Debenture issued | $ 10,310,000 | $ 10,310,000 |  |  |
Minimum |  |  |  |  |
Common equity tier 1 capital to risk-weighted assets: |  |  |  |  |
Capital conservation buffer percentage |  |  |  |  | 0.625%
Maximum |  |  |  |  |
Common equity tier 1 capital to risk-weighted assets: |  |  |  |  |
Capital conservation buffer percentage |  | 2.50% |  |  |

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